Sorry for the delay, but I had to wait for some people to be demoted.
I found essentially nothing, but to answer your questions:
1. I reviewed the mailchimp log. I can paste in the whole log if it would help but I found that on June 26th five people who had cancelled their subscriptions were demoted. Only two of them were removed from the mailchimp list. One of them happened at 12:48pm. The two that didn't work should have happened at 12:48pm, but they did not happen. There is no mention of those two in the log file at all. The fourth person was demoted and removed from mailchimp at 1:01 and the last at 1:10pm.
Is it possible that the problem is that the two that were missed had the same time stamp on them? I don't know what I could do about that, but that's the only suspicious thing I found.
2. Yes, the two people who were not removed had the same email address in wordpress as they had in mailchimp.
3. Yes, they were in the list for s2members at the "basic" membership level (it's the only level) and were demoted to "free".
I really appreciate any help you can offer. Thanks.